Porous silicon is produced very simply and inexpensively by anodic etching of monocrystalline silicon in a hydrofluoric acid solution. The porosity and microstructure of the porous silicon and thus also the optical refractive index depends, among others things, upon the doping of the starting material and the etching current density. By a change with time of the etching current density or by the use of a material with an appropriate doping profile, it is possible to produce porosity supergrids which can function as optical filters, for example, as Bragg-Reflectors or Fabry-Perot-Filters (M. G. Berger et al., J. Phys. D: Appl. Phys. 27, 1333, (1994)).
The application of such filters to an optical detector, for example, a pn-diode, changes the spectral sensitivity of the detector. The use of a number of detectors, with different filters enables, for example, color recognition or the detection of an energy spectrum of incident light with spatial resolution.